In recent years, with the reduction in weight and size of electrical appliances, development of a non-aqueous liquid electrolyte secondary battery having high energy density, for example lithium secondary battery, has been advanced. Also, as application field of lithium secondary battery is expanded, further improvement in its battery characteristics has been desired.
In this situation, a secondary battery based on metal lithium as negative electrode has been studied as a battery capable of achieving higher capacity. However, there is a problem that metal lithium grows as dendrite on repeated charges and discharges, and when this reaches the positive electrode, short circuit in the battery occurs. This has been the greatest obstacle in putting a lithium secondary battery based on metal lithium as negative electrode to practical use.
On the other hand, a non-aqueous liquid electrolyte secondary battery has been proposed, in which carbonaceous material capable of intercalating and deintercalating lithium, such as coke, artificial graphite or natural graphite, is used for the negative electrode in place of metal lithium. In such a non-aqueous liquid electrolyte secondary battery, growth of metal lithium as dendrite can be avoided and therefore, battery life and safety can be improved. When graphite of this kind is used as negative electrode, capacity is known to be usually of the order of 300 mAh·g−1, 500 mAh·cm−3.
In recent years, proposals have been made for the negative-electrode active material based on simple metal element capable of forming an alloy with lithium such as silicon (Si), tin (Sn) and lead (Pb), an alloy containing at least one of these metal elements, or metal compound containing these metal elements (hereafter referred to as “negative-electrode active material containing Si, Sn, Pb and the like”, as appropriate). The capacity of these materials per unit volume is of the order of 2000 mAh·cm−3 or larger, which is about 4 times that of graphites or even larger. Therefore, higher capacity can be obtained by using these materials.
Although a secondary battery using negative-electrode active material containing Si, Sn, Pb and the like is suitable for realizing higher capacity, there is a decrease in safety, and negative-electrode active material deteriorates on repeated charges and discharges, leading to reduced charge-discharge efficiency and deterioration of cycle performance.
Therefore, in order to secure safety and prevent a decrease in discharge capacity, a proposal has been made to include cyclic carbonate ester or a polymer of carbonate ester and phosphoric acid triester in the non-aqueous liquid electrolyte used for a secondary battery (refer to Patent Document 1). Furthermore, a proposal has been made to add, in the non-aqueous liquid electrolyte, a heterocyclic compound having sulfur atom and/or oxygen atom in the ring structure and to form a protective layer on the surface of the negative-electrode active material, thus improving charge-discharge cycle performance (refer to Patent Document 2). Moreover, another proposal has been made to add, in the non-aqueous liquid electrolyte, LiB(C2O4)2 to form a protective layer on the negative electrode, thus improving cycle performance (refer to Patent Document 3).
[Patent Document 1] Japanese Patent Laid-Open Publication No. Hei 11-176470
[Patent Document 2] Japanese Patent Laid-Open Publication No. 2004-87284
[Patent Document 3] Japanese Patent Laid-Open Publication No. 2005-228565